Conventionally, backlight illumination devices utilizing a cold cathode fluorescent tube have been widely used in liquid crystal display devices using liquid crystal display panels. In recent years, attention has been focused on backlight illumination devices using three color light emitting diodes (LED devices) of red, green and blue lights for the reproduction of more clear and natural color tones, and the development thereof has been vigorously promoted.
A planar illumination device of the lateral light source type so-called edge light type in which light from a light source is incident through a side surface (light incident surface) of a light guide plate and light is emitted from one principal surface (light output surface) of the light guide plate for illumination is used as a backlight illumination device with a relatively small size. On the other hand, a direct illumination device in which cathode fluorescent tubes or LED devices are arranged in a planar manner is used for backlight illumination requiring a large size and a high luminance.
A demand for liquid crystal display devices with thin and large screens such as wall mounted TVs is thought to increase in the future. However, in order to realize this, direct illumination devices have a problem of being difficult to be thinned and edge light type illumination devices using conventional light sources have a problem of being unable to ensure a sufficient luminance if screens are large.
To realize a liquid crystal display device with a thin large screen, researches have started on an edge light type backlight using a laser light source, which provides high luminance and which is suited for a high power output.
Furthermore, in order to realize a still higher luminance and lower power consumption, the methods for better utilization of a backlight illumination has been considered. For example, Patent Document 1 discloses a method improving the light utilization efficiency of a liquid crystal display device by providing LEDs with polarization anisotropy.
However, the foregoing conventional structure of Patent Document 1 has the following drawbacks. That is, according to Patent Document 1, since a light is incident on a light guide plate from only one direction, a problem of non-uniform luminance is liable to occur when applied to a large size screen. A problem of non-uniform color is also liable to occur due to differences in absorption when three color lights, i.e., red, green and blue lights propagate in the light guide plate.
As described above, the enlargement of thin flat displays represented by plasma displays and liquid crystal displays have been promoted at a rapid pace in recent years. A direct illumination device in which cathode fluorescent tubes are arranged in a planar manner has been conventionally used as a backlight of a liquid crystal display device requiring a large size and a high luminance. Power consumption thereof tends to increase substantially in proportion to the screen size. Furthermore, the power consumption of the backlight accounts for the large proportion of the total power consumption of the liquid crystal display device, and the problem of the power consumption has been a curtail issue for the liquid crystal display device.
In recent years, attention has been also focused on backlight illumination using light emitting diodes (LED devices) of three primary colors for the reproduction of more clear and natural color tones. Incidentally, a planar illumination device of the lateral light source type, a so-called “edge light type” has been used as a conventional backlight illumination device with a relatively small size wherein a light emitted from a light source is incident on the light guide plate through a side surface thereof and a light is outputted from one principal surface of the light guide plate to be used for illumination. Here, an attempt has been made to apply the foregoing edge light type illumination device to a thin large screen by adopting high-output laser light sources. However, such applications adopting the light-output laser light sources have a drawback in that the required power consumption is larger than that of cathode fluorescent tubes at present, and a reduction in power consumption is therefore a critical issue.
In response, various methods have been proposed to realize a reduction in power consumption. examples of which includes the method of reducing the power consumption by controlling the backlight luminance by limiting the maximum luminance of the backlight, or the method of reducing the power consumption by improving the utilization of the backlight luminance utilizing polarized lights (for example, Patent Document 1).
However, there still exists a strong demand for a reduction in power consumption, and with the foregoing conventional methods, the power consumption cannot be reduced to a sufficient level to meet such demands.
Patent Document 1:
    Japanese Unexamined Patent Publication No. 2006-40639